1. Field of the Invention
The present invention concerns a stepping motor driver for controlling rotational angular position and rotational speed of a rotor of a stepping motor.
2. Description of the Prior Art
With the high functionalization of systems equipped with motors, motors of which noise levels and vibration levels are low, and of which rotational speed ranges are wide, are being demanded. A stepping motor has excellent properties such as a small rotational angular position error and a wide rotational speed range. However, the stepping motor is caused to make a stepping rotation by changing instantaneously an excitation current for windings at each time when an external command pulse is given. Accordingly, there have been problems that the stepping motor causes vibration and noise and that it tends to step out when the excitation current is changed.
To solve such problems, a micro-step excitation system in which an inverter of a PWM (pulse width modulation) type is used to smoothly change the excitation current for windings is being generally used.
In the micro-step excitation system being used in general, a current of a sinusoidal wave form is divided to form stepped currents having different levels, and the stepped currents are fed to the respective motor windings, with phase differences between them according to the number of phases of the motor. Accordingly, it is necessary to control phase currents according to the number of phases of the motor. So, the construction of the driver becomes complicated with the increase of the number of phases of the motor.
To simplify the construction of the driver, it is conceived to apply an inverter controller technique that is generally used for controlling an AC servomotor, and in which rotational coordinate system is introduced, to the stepping motor driver.
The stepping motor driver to which the inverter controller technique is applied is suitable for realizing a micro step drive of the motor at a high resolution, since a rotational angle can be controlled directly.
As an example of such stepping motor driver, there is a stepping motor controller disclosed in Unexamined Japanese Patent Publication No.6-225595. In this driver, each phase current in a fixed coordinate system is converted into a phase current in a d-q rotational coordinate system in which d-axis is in a direction of a magnetic flux of a rotor and q-axis is in a direction perpendicular to the d-axis, and the control of the currents are dealt with in the rotational coordinate system. An encoder is connected to a stepping motor, and closed loop control systems for current control, for velocity control and for position control are composed. Each information in the fixed coordinate system is transformed into information in the d-q rotational coordinate system to carry out the position control.
Taking a current in the d-axis direction to be an exciting current and a current in the q-axis direction to be a torque current, a command current in the q-axis is varied to generate a torque for rotating the rotor. The current is varied so as to make the current in the q-axis direction minimum when the rotational position of the rotor reaches a desired position.
To simplify the construction of the controller, a non-interference component is omitted and the command current is given directly on the d-axis or q-axis. In this controller, since the rotational position of the rotor of the motor is controlled so as to make a position detection signal coincide with a position command, a micro step drive of the stepping motor in accordance with the resolution of a position controller and that of a position detector is possible.
In the stepping motor controller disclosed in the Unexamined Japanese Patent Publication No.6-225595, the position detection signal of the rotor detected by the position detector is used for controlling rotational angular position of the rotor of the motor. That is, a current in the motor is transformed into a current in the d-q rotational coordinate system in which the d-axis is in a direction of the magnetic flux of the rotor and the q-axis is in a direction perpendicular to the d-axis, using the position detection signal detected by the position detector. The control in the stepping motor controller is a closed loop position control using the position detection signal detected by the position detector.
Further, a current perpendicular to the magnetic flux, that is, a current in the direction of the q-axis, is controlled according to a velocity deviation to control the torque to be generated. Accordingly, the position controller and a velocity controller are required to be provided to effectuate the position control.
Thus, there is a problem that the construction of the stepping motor controller such as disclosed in the Unexamined Japanese Patent Publication No.6-225595 is complicated, and consequently, expensive.
It is an object of the present invention to provide a stepping motor driver having a simple construction, low in price and having a micro step drive function with high degree of accuracy.
In order to achieve the above object, the present invention provides a stepping motor driver comprising;
current sensors that detect respective winding current values of corresponding phases of a stepping motor that is caused to make a stepping rotation at each time when an external command pulse is applied,
an angle detection means that detects a rotor angle value indicative of a rotational angular position of a rotor, of the stepping motor, comprising a permanent magnet,
a first coordinate transformation means that receives the winding current values from the current sensors, receives the rotor angle value from the angle detection means, and transforms the winding current values into winding current values in a rotational coordinate system,
an angle calculation means that receives a command pulse train composed of the external command pulses and calculates a command angle value,
a third coordinate transformation means that transforms command current values given from the outside into command current values in the rotational coordinate system according to the difference between the command angle value and the rotor angle value,
a current control means that generates current control signals in the rotational coordinate system based on the differences between the command current values in the rotational coordinate system and the winding current values in the rotational coordinate system,
a second coordinate transformation means that receives the current control signals in the rotational coordinate system from the current control means, receives the rotor angle value from the angle detection means, and transforms the current control signals in the rotational coordinate system into current control signals in a fixed coordinate system, and
a current supply means that receives the current control signals in the fixed coordinate system from the second coordinate transformation means, and supplies currents to windings of respective phases of the stepping motor.
According to the present invention, it is possible to provide a stepping motor driver having a simple construction, low in price and having a micro step drive function with high degree of accuracy, since the winding current values are transformed into the values in the rotational coordinate system, the current control signals are generated based upon the differences between the command current values and the winding current values in the rotational coordinate system, the current control signals in the rotational coordinate system are transformed into the current control signals in the fixed coordinate system, and then fed to the current supply means.